It is known in the art to provide resonators that comprise membrane or film layers. By example, in an article entitled "Acoustic Bulk Wave Composite Resonators", Applied Physics Lett., Vol. 38, No. 3, pp. 125-127, Feb. 1, 1981, by K. M. Lakin and J. S. Wang, an acoustic bulk wave resonator is comprised of a thin film piezoelectric layer of Zinc-Oxide (ZnO) sputtered over a thin membrane of Silicon (Si). The resonator exhibits high acoustic reflectivity characteristics at interfaces between the air and the device, therefore enabling the device to have a suitable figure of merit (Q). Notwithstanding the beneficial characteristics of the device, the fabrication of resonator filters comprising thin membranes is a cumbersome process, requiring, by example, the deposition of the membrane layer and then the performance of photolithographic steps.
In view of these problems, resonators have been fabricated which incorporate so called "acoustic mirrors" instead of membranes. An example of one these devices is disclosed in the article entitled "Ultrasonics in Integrated Electronics", Proc. IEEE, Vol. 53, October 1965, pp. 1305-1309, by W. E. Newell. For these types of resonators, the acoustic mirror may comprise a lower layer having a low acoustic impedance and a thickness of, by example, one-quarter wavelength, and an upper layer having a high acoustic impedance and a high reflectivity characteristic. The lower layer functions as an "impedance converter" since it can transform the acoustic impedance of a substrate to a very low value. For a case in which each of the layers has a thickness of one-quarter wavelength, the conversion factor of the pair of layers is equal to the square of a ratio of their respective impedances.
Another example of a device incorporating an acoustic mirror structure instead of a membrane may be found in an article entitled "Development of Miniature Filters for Wireless Applications", IEEE MTT-S Digest, 1995, pp. 883-886, by K. M. Lakin, G. R. Kline, and K. T. McCarron.
Unfortunately, because many layers need to be formed to create these types of devices, it can be difficult to form the layers to have precise "design" thicknesses. Also, during the fabrication of these resonators the process of sputtering the layers can consistently result in the layers having incorrect thicknesses. A further problem with these types of resonators is that the intrinsic stress of the layer materials forming the resonators can inevitably strain the lower stack layers, eventually resulting in at least one of these layers being peeled from the substrate. This problem becomes more severe for resonators having thicker layer stacks.
Another article is entitled "Temperature Compensated High Coupling and High Quality Factor ZnO/SiO.sub.2 Bulk Wave Resonators on High Resistance Substrates", IEEE 1984 Ultrasonics Symp., pp. 405-410, by T. Shiosaki, T. Fukuichi, M. Tokuda, and A. Kawabata. This article discloses a bulk wave resonator that includes an insulating silicon-dioxide (SiO.sub.2) film. The device eliminates an influence of a parasitic parallel branch formed between a top electrode and a bottom electrode of the resonator.